High temperature tribological behavior of W-DLC against aluminum

Diamond-like carbon (DLC) coatings are well suited for applications that require minimum adhesion and low coefficient of friction (COF) against aluminum alloys. These properties however deteriorate rapidly at elevated temperatures, and coating wear occurs. In this study, tribological behavior of W containing DLC (W-DLC) were studied as a function of testing temperatures up to 500 °C, and the sliding-induced surface and subsurface damage at these temperatures was investigated. Pin-on-disk tests performed on W-DLC run against 319 Al showed a low COF of 0.2 at 25 °C, whereas between 100 °C and 300 °C, a high average steady-state COF of 0.60 was recorded. At 400 °C the COF decreased to 0.18, and this reduction in COF continued with increasing the temperature to 500 °C (0.12). It was observed that the formation of transferred material layers on 319Al was the governing mechanism for the low COF. The Raman analysis revealed that at room temperature these layers were rich in carbon, whereas at 400 °C the transfer layers consisted of tungsten oxide. According to transmission electron microscopy (TEM), and X-Ray photoelectron spectroscopy (XPS), of the coatings tested at 400 °C and 500 °C a thin (20 nm) tungsten oxide layer was formed on their top surface. This in turn led to the formation of tungsten oxide rich transfer layers that is believed to reduce the COF at temperatures above 400 °C.